Method of reducing curl in making a continuous web of paper

ABSTRACT

The method involves the use of upper and lower drying sections, each comprising a plurality of steam-heated drying rolls, which are employed in the drying of the paper web as it passes therebetween. A first control loop is associated with the upper section and a second control loop is associated with the lower section. These dryer loops determine the respective amounts of steam delivered to the two sections. A moisture control loop outputs a set point to the first dryer loop and a second set point is provided for the second dryer loop, and the second set point being modified so that the rate of steam flow to the second dryer section is maintained in a preferred relationship to the rate supplied by the first dryer section so that the curl of the web is minimized.

United States Patent Keyes, IV et a].

[451 Apr. 25, 1972 [54] METHOD OF REDUCING CURL IN MAKING A CONTINUOUS WEB OF PAPER 2 Marion A. Keyes, IV, South Beloit, Ill.; John A. Gudaz, Beloit, Wis.

U.S. Cl ..l62/l97, 34/48, 162/198 Int. Cl. ..D2lf 5/06 Field of Search 1 62/197, 270, 198, 263; 34/48 Inventors:

Assignee:

[56] References Cited OTHER PUBLICATIONS Goldner, P. Drying Systems for Curl Control Tappi Vol. 47

No. 7 (July 1964) p. 168A- 170A Spitz et al. The Cause and Cure of Paper Curl" Tappi Vol. 46 No. 11 (Nov. 1963) p. 676- 680 Primary Examiner-S. Leon Bashore AssistantExaminer-Alfred DAndrea, Jr. Attorney-Bugger, Peterson, Johnson & Westman [5 7] ABSTRACT The method involves the use of upper and lower drying sections, each comprising a plurality of steam-heated drying rolls, which are employed in the drying of the paper web as it passes therebetween. A first control loop is associated with the upper section and a second control loop is associated with thelower section. These dryer loops determine the respective amounts of steam delivered to the two sections. A moisture control loop outputs a set point to the first dryer loop and a second set point is provided for the second dryer loop, and the second set point being modified so that the rate of steam flow to the second dryer section is maintained in a preferred relationship to the rate supplied by the first dryer section so that the curl of the web is minimized.

6 Claims, 4 Drawing Figures P/l 59TRANSDUCER CALENDER STACK F/RST DRYER W57 SECTION HEAD- 50X J SfCONDARY DRYER SECT/O/V P// /6/7AL ,c M/PZLL y u TRANSDUCER AVHAGE L72 r/m/s o ucm mu 511cm COMPl/Tf/Q 58b 60 MOISTURE l PRL'SSURE cwvmmm co/vmoum 54 54c C1 1 Q2" I 0 SET PU/N 55A 1 6 RAT/0 I CALCULATOR 55 we 55: r 8 74 RA 770 RAT/0 PRESSURE srr P0/NT| co/vmoum CONTROLLER Patented April 25, 1972 3,658,642

2 Sheets-Sheet 2 FROM F/G. Z MOISTURE con/mama? 56 l I I mac 765C 16 5 U5 TRACTOR g I 650 Ljbq l 64 DIP/THENCE D/FFEAE/VCE 4 AJ/awe 5E7 PO/NT l CONTROLLER I JUNCT/ON 1661b i660 166 ym FROM 3 MO/STURE co/vmoum 56 I RAT/0 63 I I 2650 I 64 m MULT/PL/[R SUM/WW6 5157 Pom/r I v JUNCTION 70 FROM G 4 MO/STUAE CONTROLLER 55 5 355q I 4 T0 DIFFERENCE I SUBTRACTO/P SUM/WING DO/N7 JUNCTION 70 :365b {555 355C:

METHOD OF REDUCING CURL IN MAKING A CONTINUOUS WEB .OF'PAPER CROSS REFERENCE TO RELATED APPLICATION This application is a division of our co-pending application, Ser. No. 784,665, filed Dec. 18, 1968, now US Pat. No. 3,564,724 for MOISTURE CONTROL SYSTEM WITH CURL COMPENSATION.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to paper-making,and pertains more particularly to a method for minimizing the amount of curl in the web.

2. Description of the Prior Art It is generallyknown that curl or web curvature is caused by several factors. Orientation of the fibers throughout the web and internal stresses within the web are two factors. An important third factor, however, is the unequal distribution of moisture within the web itself. For example, if the web is drier adjacent its top surface than its bottom surface, there is a shrinkage at the top that causes the web to curl upwardly. Compensation for curling, whether up or down, is necessary on paper grades, such as tab card stock, and any other situation where uniform sheet characteristics are critical.

In some prior art instances, the-curl has been ignored and the paper has been of inferior quality. However, where the use of the paper demands that curl be corrected for, it has proved exceedingly difficult to introduce a compensation that will assure uniformity in the final product. The manner of correction in the past has been largely left up to the judgment of the individual operator and his personal previous experience. In some instances, the operator has made a correction at one drying stage that proves excessive by the time the paper web reaches the reel end.

' SUMMARY OF THE INVENTION that one section contributes more heat'than the other when the tendency to curl requires differing drying rates. In two embodiments, a ratio of the pressure that determine the drying rates is utilized and in two additional embodiments, the difference between the pressures is resorted to. The set points representing the ratios of pressures and the difference pressures, respectively, are obtained by initially observing the data and selecting the set points so that optimum results will occur. When the necessary amount of correction has been realized, it is then known that the correct ratio or correct difference as far as set points are concerned have been entered by the operator and from that time on the ratio or difference will be automatically maintained.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a typical paper making machine with the curl compensation obtained by means of a ratio loop in association with other control loops;

FIG. 2 depicts a modification of the invention, a difference control loop being substituted for the ratio control loop of FIG. 1;

FIG. 3 is another modification of the invention in which a multiplier circuit is substituted for the ratio control loop of FIG. I and FIG. 4 is a fragmentary view illustrating still another modification that the invention may assume, the instant embodiment making use of subtraction rather than a ratio.

DESCRIPTION OF TI-IE 'PREFERRED EMBODIMENT A conventional paper making machine has been indicated generally by the reference numeral 10. The machine "IO-comprises the usual headbox II which distributes'the' pulp stock onto a Fourdrinier wire 14. The web 16 thus formed is directed through a wet press section 18 into a first .dryer section 20. From the first dryer-section' 20 the web16 proceeds to a size press22 where an appropriate sizing additive is applied when such an agent is required. From' the size press 22 the web 16 continues to a secondary dryer section124 which includes what are sometimes referred to as upper and lowertiers but which will herein be referred to as upperand lower dryer sections, these actually being subsections of thedryer section '24. The upper section has been labeled 24a and is composed of any preferred number of steam-heated drying rolls, whereas the lower section has been designated'by the numeral 24b. These tiers or sections 24a and 24b are important to a practicing of the present invention as will soon become clear.

When calendering is desired, a calender stack 30 is employed. Thereafter, a moisture gauge 32 senses the amount of moisture contained'in the web 16 as the web is wound onto a take-up reel 34. The moisture gauge 32 may be of the scanning variety. Actually, the particular type of gauge is unimportant to an understandingof the invention.

A source or supplyof steam for the various drying sections has been indicated by the reference numeral 36. A valve 38 is connected to the source of supply 36 and varies the amount or rate of flow of steam passing to the upper section 24a. A valve 40 supplies steam to the lower section 24b. A current-to-pressure-transducer 42 positions the valve 38 as a means to regulate the amount-of steam delivered to the upper section 240, whereas a similar transducer '44 positions the valve 40 as a means to regulate the amount of steam fed to the lower drying section 24b.

The system for controlling the transducers 42 and 44 has been generally designated by the reference numeral 50. As explained above, the moisture gauge 32 may be of the scanning type. Thus, the gauge 32 may be considered as traversing the web 16 and in this way obviates the probability of a control predicated on a very atypical point of the web 16. However, the system will operate on a single point as long as the single point is representative of the average moisture. However, assuming that a representative single point cannot be reliably obtained, a valid average computer 52 has been depicted having a first input line 52a connected to the moisture gauge 32 and a second input line 52b which receives a digital control signal only when the gauge 32 has completed a scan from front to back or vice versa'.

The output line leading from the valid averagecomputer has been labeled 52c and is connected to an input terminal 540 of a summing junction or comparator 54. Hence, the terminal 54a introduces one input, this being the computed valid average moisture signal, into the summing junction 54. Of course, if a good moisture profile exists, the valid average need not be calculated and a single point would be sufficiently representative of the moisture content of the web 16' so as to serve as the input to the summing junction 54.

The summing junction 54 has another input terminal 54b which carries the moisture set point which is representative of a desired moisture level that should exist in the web 16 just prior to the winding of the web onto the reel 34. The summing junction 54 has an output terminal 54c and any difference between the signal applied via the input terminal 54a and the moisture set pointapplied via the terminal 54b will appear as an error signal at the outputterminal 540.

The output signal from the terminal 540 is fed to a moisture controller 56. The moisture controller then outputs a set point or a signal which functionsas one of the inputs to a summing junction or comparator 58, being impressed upon one input terminal 58a belonging to this particular summing junction. Actually, itis the signal from the moisture controller 56 that constitutes a set point for a control loop for the upper drying section 24a. The set point derived from the moisture controller 56 is compared with a measured pressure signal coming from a pressure-to-current transducer labeled 59 which receivesa pressure signal indicative of the actual steam pressure existing at the upper dryer section 240.

Any difference between the incoming signals applied to the terminals 58a, 58b is outputed as an error signal on the terminal 58c which is connected directly to a pressure controller 60. Thus, the pressure controller 60 operates on the error transmiteed from the summing junction 58 and this error, which is a time function, determines to what extent the valve 38 should be repositioned, doing so through the current-topressure transducer 42. In other words, the valve 38 is adjusted so as to vary the rate of steam flow to the upper dryer section 24a in accordance with the value of the error signal that is delivered to the pressure controller 60.

Mainly to enable a ready comparison of FIG. 1 with FIGS. 2-4 the moisture controller 56, which outputs a signal used in the control of the upper dryer loop just described, is connected to circuitry generally denoted by the reference numeral 61 by a line 62 that extends thereto from the moisture controller 56. In the specific situation being described, the circuitry 61 constitutes a calculation ratio loop. To further facilitate a comparison with FIGS. 2-4 it will be observed that a line 63 is utilized leading into the circuitry 61 and a line 64 extending therefrom. The above-mentioned line 62 leads to an input terminal 65a belonging to a set point ratio calculator 65. It is the function of the set point ratio calculator 65 to produce a signal indicative of the ratio of two set points, one set point being the set point provided by the moisture controller 56 and the other set point being the set point utilized in a control loop for the lower dryer section 24b. It is via an input terminal 65b leading into the set point ratio calculator 65 that enables a comparison to be made of this particular set point with the set point applied to terminal 65a. The calculator 65 has an output terminal 65c.

The division performed by the calculator 65 results in the forwarding of a signal via the output terminal 650 which represents the ratio of the two set points applied to the input terminals 650 and 65b. The output terminal 65c is connected to an input terminal 66a belonging to a summing junction or comparator 66, the summing junction 66 having a second input terminal 66b which is connected tothe previously mentioned line 63. In the present instance, it is the line 63 that carries a ratio set point which is a signal indicative of a desired ratio that should be maintained between the set points applied to the terminals 65a and 65b. Any difference between the signals impressed on the terminal 66a and 66b appears as an error signal on the output terminal 66c. In this way, a ratio controller 68 receives a signal that represents the difference between the inputs on the terminals 660 and 66b. The ratio controller 68 then provides a set point having a desired relationship with the set point derived from the moisture controller 56 which last-mentioned set point is utilized in a control loop for the lower dryer section 24b.

Included in the control loop for the lower dryer section 24b is a summing junction or comparator circuit 70 having a pair of input terminals 70a, 70b and an output terminal 70c. Whereas the input terminal 70a is connected to the ratio controller 68, the input terminal 70b is connected to a pressureto-current transducer 72 that forwards an electrical signal indicative of the actual steam pressure existing at the lower dryer section 24b. It is the difference between the inputs applied to the terminals 70a and 70b that is differenced at the summing junction 70 to produce an error signal at the output terminal 700 which is fed to a pressure controller 74. Through the agency of the pressure controller 74 and the current-topressure transducer 44 the valve 40 is positioned to whatever extent is necessary to adjust the steam flow to the dryer section 24b.

Although the operation that occurs has been presented as the written description has progressed, nonetheless it will be of benefit, it is believed, to recapitulate briefly as to what transpires. Hence, a moisture control loop is provided by means of the moisture gauge 32, the computer 52, the summing junction 54 and the moisture controller 56. Two pressure control loops are provided, as hereinbefore alluded to. The first one which controls the upper dryer section 24a include the transducer 59, the summing junction 58, the pressure controller 60 and the transducer 42 for adjusting the valve 38. The other pressure control loop includes the trans ducer 72, the summing junction 70, the pressure controller 74 and the transducer 44 for adjusting the valve 40. Set points are provided for each of these pressure control loops. The set point for the upper loop is derived directly from the moisture controller 56. On the other hand, the set point for the lower dryer section is dependent on the upper loop set point as influenced by the ratio set point that is determined by observing the resulting curl condition of the web 16 as it is wound onto the reel 34. If the initial value of the ratio set point that is adopted proves inadequate, another value is chosen. Once the ratio set point is properly determined, though, then the system 50 continues to produce a paper web 16 having a minimum degree of curl. Thus, the valve 40 will control the rate of steam flow to the lower section 24b in a predetermined relationship with the rate of steam flow to the upper dryer section 24a as determined by the position of the valve 38. In this way, one side of the web 16 can be dried at a different rate having preferred relationship with the other side, thereby producing a web having optimum curl characteristics imparted thereto.

Referring now to FIG. 2, it will be appreciated that the block 161 replaces the block labeled 61 in FIG. 1. In this embodiment, a differencing of the set points for the two pressure control loops is obtained. Thus, instead of a ratio control loop as set forth in FIG. 1, FIG. 2 deals with a difference control loop. Hence, the previously mentioned line 62 leads to a first input terminal 165a belonging to a differencing means or subtractor 165 which has a second input l65b. Thus, the subtractor 165 produces an output on the terminal l65c which represents the difference between the two dryer pressure set points.

Included in the block 161 is a summing junction or comparator 166 having a first input terminal 166a connected to the output terminal 165s and a second input terminal I66b connected to the line 63. It is via the line 63 in the present instance that a difference set point is applied to the input terminal 166b and this is differenced with the actual difference signal arriving via the input terminal 166a to provide an error signal representative of any disparity. The error signal is fed from the terminal 1660 to a difference controller 168.

Hence, the set point provided by the difference controller 168 is fed back by means of the terminal 165b and is subtracted from the value of the set point impressed upon the input terminal 165c which set point is derived from the moisture controller. Otherwise, the operation is the same as that provided by the system depicted in FIG. 1 labeled 50, the circuitry shown in block 161 being substituted for the circuitry ofblock 161 in FIG. 1.

Turning now to a brief description of FIG. 3, it will be perceived that this figure embodies a simplification of FIG. 1 as far as block 61 is concerned. To orient the reader, a block labeled 261 is presented. However, the block 261 contains only a multiplier 265 having an input terminal 265a and an input terminal 265b plus an output tenninal 2650. The same set point provided by the moisture controller 56 arrives via the line 62 and is inputed at the terminal 265a. The same ratio set point utilized in FIG. 1 is applied to the input terminal 265a. The two set points are multiplied together to provide the second pressure set point for the lower control loop which includes the dryer section 24b. The operation is otherwise the same as that of the system shown in its entirety in FIG. 1.

The remaining modification is presented in FIG. 4. This arrangement may be considered as a simplification of FIG. 2. In this instance the replacement for the block 61 of FIG. 1 has been labeled 361. As with FIG. 2, the present modification operates on a difference between the set points rather than a ratio thereof. It will be discerned that the subtractor labeled 365 has an input terminal 365a and an input terminal 365k. it is the input terminal 365:: that is connected to the line 62 which is in turn connected to the output sides of the moisture controller 56. Hence, the same set point utilized in FIGS. 1-3 is employed in the present situation. The set point representative of a desired difference between set point values is the same as that utilized in FIG. 2, the set point being applied via the line 63 and thus introduced to the subtractor 365 through the agency of its second input terminal 365b. The difference between the two inputed set points appears as an output on the terminal 3650 which serves as the set point for the control loop associated with the lower dryer section 24b. In this instance, not only is a subtraction operation being performed in contradistinction to the ratioing of two set points but it is done without feedback as resorted to in FIG. 2. Otherwise, the functioning of the modification succinctly presented in FIG 4 is the same as the other situations herein described.

We claim:

1. The method of reducing curl in apparatus for making a continuous web of paper comprising the steps of directing heat against both sides of said web to reduce the moisture content of said web to a desired level, initially varying the amount of heat applied to said one side of said web in relation to the amount of heat applied to the other side until a desired reduction in the curl of said web has been produced, the relationship of the amount of heat applied to said one side of said web with respect to the amount of heat applied to the other side thereof which produces said desired reduction in curl thereby establishing a preferred relationship between the heat applied to said one side and the heat applied to said other side and thereafter varying the amount of heat applied to said one side of said web with respect to the heat applied to the other side in accordance with said predetermined relationship while substantially maintaining the desired moisture content of said web at said preferred level of moisture in said web.

2. The method of claim 1 in which one of said sides faces upwardly and the other of said sides faces downwardly, the heat applied to said upper side being varied in accordance with the amount of moisture contained in said web to maintain the moisture content of said web at said desired level and the amount of heat applied to said lower side being varied in said preferred relationship to that of said upper side whereby a reduction in the curl of said web is maintained.

3. The method of claim 1 in which the heat applied to said one side is varied in accordance with a ratio of the difference between the desire moisture for said web and the actual moisture in said web with respect to a desired ratio to provide said predetermined relationship with respect to the heat applied to said other side of said web.

4. The method of reducing curl in the making of a continuous web of paper comprising the steps of directing heat against one side of said web in accordance with the moisture contained in said web after at least some drying thereof has been accomplished to maintain a preferred level of moisture in said a web, directing heat against the other side of said web in relationship with the heat directed to said one side to initially reduce the curl of said web, adjusting the relationship of the amount of heat directed to said other side of the web with respect to the amount of heat being directed to said one side of the web until a desired reduction in the degree of curl of said web has been effected, and thereafter maintaining said relationship of the amount of heat directed to the other side of said web with respect to the amount of heat directed to said one side of the web while substantially maintaining the desired moisture content of said web at said preferred level of moisture in said web.

5. The method of claim 4 in which said relationship of the heat directed against said other side with respect to the heat directed against said one side is varied until said relationship effecting said desired reduction in curl is achieved, said relationship constituting a preferred relationship of the heat directed to said other side with res ect to the heat directed to said one side and said preferre relationship lS thereafter maintained while maintaining said moisture content at said desired level.

6. The method of claim 5 in which the heat directed against said other side is varied in accordance with a ratio of the difference between the desired moisture for said web and the actual moisture in said web with respect to a desired ratio to provide said predetermined relationship to the heat directed to said one side.

0' I I IIK 

2. The method of claim 1 in which one of said sides faces upwardly and the other of said sides faces downwardly, the heat applied to said upper side being varied in accordance with the amount of moisture contained in said web to maintain the moisture content of said web at said desired level and the amount of heat applied to said lower side being varied in said preferred relationship to that of said upper side whereby a reduction in the curl of said web is maintained.
 3. The method of claim 1 in which the heat applied to said one side is varied in accordance with a ratio of the difference between the desire moisture for said web and the actual moisture in said web with respect to a desired ratio to provide said predetermined relationship with respect to the heat applied to said other side of said web.
 4. The method of reducing curl in the making of a continuous web of paper comprising the steps of directing heat against one side of said web in accordance with the moisture contained in said web after at least some drying thereof has been accomplished to maintain a preferred level of moisture in said web, directing heat against the other side of said web in relationship with the heat directed to said one side to initially reduce the curl of said web, adjusting the relationship of the amount of heat directed to said other side of the web with respect to the amount of heat being directed to said one side of the web until a desired reduction in the degree of curl of said web has been effected, and thereafter maintaining said relationship of the amount of heat directed to the other side of said web with respect to the amount of heat directed to said one side of the web while substantially maintaining the desired moisture content of said web at said preferred level of moisture in said web.
 5. The method of claim 4 in which said relationship of the heat directed against said other side with respect to the heat directed against said one side is varied until said relationship effecting said desired reduction in curl is achieved, said relationship constituting a preferred relationship of the heat directed to said other side with respect to the heat directed to said one side and said preferred relationship is thereafter maintained while maintaining said moisture content at said desired level.
 6. The method of claim 5 in which the heat directed against said other side is varied in accordance with a ratio of the difference between the desired moisture for said web and the actual moisture in said web with respect to a desired ratio to provide said predetermined relationship to the heat directed to said one side. 